The use of more than one vibration generator associated with vibratory material handling apparatus to provide selectable conveying directions is not new. For example, Musschoot U.S. Pat. Nos. 2,712,459, 3,793,780 and 4,025,419 show systems wherein electric motors rotate shafts each carrying an eccentric weight, with the motors being attached directly to the vibrating apparatus. The previous systems shown in said patents were "brute force" systems requiring heavy duty motors to generate the vibratory forces necessary. Tuned exciter natural frequency systems were not used in such multi-vibration generator arrangements perhaps for the reason that such natural frequency spring exciter systems would continue to vibrate with the material-carrying part of the apparatus even though the vibration generator associated therewith had been de-energized.
Referring again to prior apparatus in this field, it has been customary in the past to remove foundry sand from a casting through what is known in the industry as a shakeout screen. Basically, this comprised a grating mounted for vibratory movement and the sand and casting were placed thereon with the vibrations serving to remove the sand from the casting. As foundry sand is expensive, the sand so removed was rendered reusable by treating it in a separate lump breaking apparatus, often consisting of apparatus similar to that shown in the Musschoot U.S. Pat. No. 4,025,419 referred to above.